1. Field of the Invention
The present invention relates to video signal processors, and in particular, video signal processors for receiving and converting a video signal having multiple bits per pixel and representing a continuous tone video image to a facsimile signal having a single bit per pixel for transmission to and reception by a facsimile machine for producing a hard copy representation of the continuous tone video image.
2. Description of the Related Art
As the sophistication and capabilities of video system components such as video cameras and tape recorders have increased and their costs have decreased, uses for such components to capture and retain visual images in the form of video signals have increased in both number and form. Two uses in particular have become substantially more widespread. One use involves the capture and retention of visual images for use at a later time. Video signal recorders, such as video tape recorders, video cassette recorders or video disks, have served quite well for such uses. Another use involves the capture and transmission of video images for use at a distant, e.g. remote, location. This type of use has generally required some means of signal transmission to convey the video signal representing the visual image to the remote location. Such means of signal transmission typically include the use of some form of hard-wired video signal transmission medium, such as co-axial cable, or a radio frequency ("RF") transceiver. The former if often unwieldy or impractical, particularly over long distances, while the latter is often expensive and subject to restricted and heavily regulated RF spectrum allocations.
Other means for conveying video signals which has been used with some success are telephone networks. By converting the subject video signal to a digital video signal consisting of video pixel data and coupling it onto a telephone line via a modulator-demodulator ("MODEM"), the video information can be transmitted, albeit slowly, to many possible locations. At the receiving end, the video pixel data can be retrieved with another MODEM and processed as needed for viewing on a video monitor or storage on video tape. Alternatively, the video pixel data, if transmitted in accordance with an appropriate data standard, can be received by a facsimile machine and "reproduced" in the form of a hard copy printout.
However, such "reproduction" by a facsimile machine is not accurate. A video signal representing a continuous tone video image, when digitized, contains video pixel data (e.g. eight bits) representing the gray-scale values, or contrast range, of the continuous tone video image. However, a facsimile machine is capable of reproducing pel data (i.e. single bit) only, which may be thought of as a single bit per pixel. Accordingly, some form of "thresholding" is often performed to convert the video pixel data to video pel data for use by the facsimile machine. However, this generally results in a reproduced video image having a flat or grainy appearance. One technique which has been used with varying success to avoid this flat image appearance is "dithering." In "dithering," for each selected group of original pixels a group of corresponding pels is produced, which as a group, has a composite gray-scale value similar to that of the original group of pixels.
Accordingly, it would be desirable to have a video-to-facsimile signal converter for receiving and converting a video signal representing a continuous tone video image to a facsimile signal suitable for transmission to and reception by a commercial facsimile machine for more accurately "reproducing" the continuous tone video image by way of a hard copy printout.